Fully differential amplifiers are widely used in modern integrated circuits in view of their relatively large output swings and relatively low susceptibility to common mode (CM) noise. One significant disadvantage of these circuits is the need for a common mode feedback (CMFB) circuit to control the CM output voltage. The main purpose of the CMFB circuit is to sense the output CM voltage and use negative feedback to force it equal to the desired common mode, or reference voltage (VREF). In some instances, however, the CMFB loop is ineffective due to large positive feedback produced by the external network overwhelming the negative feedback loop, leading to the output staying at the rails, a condition known as “latching”. Conventional solutions require a large bandwidth CMFB loop in order to avoid latching states, causing practical problems in implementing high-speed circuits.
An example representative of methods known in the arts for detection and feedback of the common mode correction signal applicable to a two-stage amplifier system is shown in FIG. 1 (prior art). A two stage Miller compensated amplifier is depicted with a traditional resistor averaged CMFB arrangement. Examination of the circuit reveals that only a fraction of the total current is controlled by the common mode loop, i.e., (I3/(I3+I4)). The DC error in the common mode output may be described by the equation:ΔVOCM=VREF−VOCM=ΔI/(gm3*A1)  [Equation 1],where ΔI is the open-loop mismatch between the current sources (I10+I20−I30−I40), and A1 is the voltage gain of the common mode sense amplifier shown in broken lines in FIG. 1 (prior art). Thus, it may be observed that the error may be reduced by increasing the transconductance gain, (gm3*A1)
For a two stage amplifier, although the feedback through the external network is negative for the differential signals, it is positive for the common mode signals. In normal operating conditions, the negative feedback loop gain is much larger compared to the positive feedback loop gain, so the latter is not of much concern. The primary problem occurs during startup or during large common mode transients, in which cases the input differential pair turns off. This may result in the output swinging to the rails and latching there. Many operational amplifier systems have no built-in provision to avoid these latching states. For the design shown in FIG. 1 (prior art), a latching state occurs either when, I4>I2, or when, I3<I1. Thus, to avoid these latching states, the CMFB loop should at least control I1. It should be appreciated by those skilled in the arts that the problem of latching can exist even if the CMFB is applied to transistor M2 instead of M3, or in the case of an NMOS input stage instead of a PMOS input stage.
When addressing these and other problems with techniques known in the arts, it is generally more difficult to compensate the CMFB loop compared to the differential loop because of two additional poles in the former. Assuming removal of right hand plane (RHP) zero due to Miller compensation by the nulling resistor Rz, and continuing to refer to FIG. 1 (prior art), the common mode loop gain is described by the equation:ACM(s)=[A01+s/ωz)]/[(1+s/ωd)(1+s/ωL)(1+s/ωf)(1+s/ωcm1)(1+s/ωcm2)]  [Equation 2], where,A0=(gm3gm5gm6gm8)/[(gm6gds5(gds4+gds3)+gm5gds2gds6)(gds11+gds8)*2gm7/gm9],ωd≈A1gm3/A0CC,A1≈gm7/2gm9,ωf=gm5/(Csb5+Cdb1+Cgd1+Cdb3+Cdb4),ωcm1=4/R1(4C1+Cgs7),ωcm2=gm9/(Cgs9+Cdb9+Cgs3),ωL≈gm8/CL, ωz=1/R1C1.For Cgs7<<4C1, ωcm1≈ωz, therefore Equation 2 may be expressed in the simplified form:ACM=A0/[(1+s/ωd)(1+s/ωL)(1+s/ωf)(1+s/ωcm2)],  [Equation 3].
For the example of FIG. 1 (prior art), the differential loop bandwidth may be described by, βgm1/CC, and the common mode loop bandwidth by, A1gm3/CC. The capacitor CC is generally selected for optimal compensation of the differential loop. Since the feedback factor (β) does not affect the common mode loop bandwidth, for a design with small feedback factor, it is possible for the common mode loop bandwidth to exceed the differential loop bandwidth, A1gm3/CC>βgm1/CC, leading to problems with common mode instability. The solution widely used in the arts is to reduce the fraction of current controlled by the CMFB, i.e., to reduce I3, and thereby reduce gm3. This approach, however, leads to increased DC common mode error, as expressed in Equation 1. Additionally, if I3 is less than I1, latching remains a problem.
In two stage amplifier systems these and other problems beset efforts to control larger current in common mode loops and to avoid latching. There is a need for improved methods and systems for stabilizing the common mode feedback (CMFB) loop, while avoiding latching states, particularly in high speed fully differential two stage amplifier systems.